The TD253 is a 10 mA germanium tunnel diode from General Electric. Tek part number is 152-0154-00.
It is used in several Tektronix instruments, including the 547 and 7B70. It is specified to have a maximum capacitance of 9 pF. There is also a TD253A, which has a maximum capacitance of 5 pF and a 190 ps rise time. The TD253B has a capacitance of 2pF and a 68 ps rise time.